media: ivtv: zero-initialize cx25840 platform data

We need to zero-initialize cx25840 platform data structure to make sure
that its future members do not contain random stack garbage.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>